Well the biggest difference is aloud is an adverb, but loud is an adjective.
But there is likely some confusion between "Aloud", "Out loud","Loudly", which are all adverbs.
"Aloud" is an adverb used in contrast to saying or thinking something quietly. So usage might be
"She read her secret journal aloud to the class."
"He couldn't help but to weep aloud".
More importantly, you should know that "aloud" is generally deemed as formal, and is rarely used in conversation.
"Out Loud" is almost the same as "aloud", they can usually be used interchangeably. For example
"He couldn't help but to weep out loud"
"She read her secret journal out loud to the class."
The difference with out loud is that it can also be used to imply that it was a sudden or unexpected out burst.
So in the acronym "LOL" (laugh out loud), it is laughing audibly as opposed to what people normally do when they are reading on the internet, which is laughing in their head or very quietly.
"Out loud" is much more common than "aloud". I can't think of an instance where I would say "aloud" in conversation without sounding a bit awkward.
"Loudly" is used most often as a quantifier of something that's loud enough or too loud. Typically, it's in a negative context.
For example in a negative context:
She is playing her music way too loudly.
You are speaking too loudly in the library.
And in a positive context:
He spoke loudly so everyone could hear him.